SELECT 'NOW()' - OK, SELECT 'CLOCK_TIMESTAMP()' - ERROR

Hello,

I found strange PostgreSQL 9.3 behavior:

> select now()::timestamp, 'now()'::timestamp;
now | timestamp
----------------------------+----------------------------
2014-08-22 08:34:00.883268 | 2014-08-22 08:34:00.883268

Second column is now() in single apostrophes.

Now, I tried similar function, clock_timestamp() and get:

> select clock_timestamp()::timestamp, 'clock_timestamp()'::timestamp;
ERROR: invalid input syntax for type timestamp: "clock_timestamp()"
LINE 1: select clock_timestamp()::timestamp, 'clock_timestamp()'::ti...
^

Why is NOW() so special? Where is it documented? And why not working with
other timestamp returning internal functions?

> select version();
version

--------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.3.4 on amd64-portbld-freebsd10.0, compiled by FreeBSD clang
version 3.3 (tags/RELEASE_33/final 183502) 20130610, 64-bit
(1 wiersz)

--
Piotr Gasidło